The focus of this research is the study of the brain mechanisms in word retrieval in normal speakers as revealed by their disruption in aphasia, where these mechanisms have become damaged or dissociated. Following an examination of perceptual competition on naming objects in fluent and nonfluent aphasia, the first of a series of experiments addresses the problem of dissociation of noun and verb impairments in Broca's aphasia by adopting Luria's distinction between predicative and nominal modes of speech. The possibility is investigated that this distinction may underlie this commonly observed dissociation. The next stage of this investigation examines the role of semantic memory in word retrieval by examining patients' competence in identifying the properties of objects to be named and comparing this ability with their success in retrieving the names of these objects. The third question in the research focuses on the stages of phonological activation and implementation in word production. A model is proposed to identify the multi-staged nature of phonological activation. Predicted differences in speed of naming following from immediate versus delayed priming of picture names will be tested with the goal of distinguishing between the effects of early and late stages of phonological activation. Finally, a series of three studies using the "gating" technique examines the differences between Broca's and Wernicke's aphasics by comparing their response to priming with word onsets and with the prosody (stress pattern and duration) of target words. This research program promises clinical insights that will be useful in the differential diagnosis of aphasia as well as making theoretical contributions to understanding the mechanisms of word production and phonological realization.